The present invention is directed to a system and a method for continuously and economically producing Dye-Sensitized Solar Cells (DSSCs) through a roll-to-roll process.
Concerns about depletion of natural resources, the stability of thermal and atomic electric power generation systems and their impact on the environment gave rise to intensive global research efforts for developing systems utilizing solar and wind energies, which are relatively typical environmental friendly energies.
Particularly, solar energy that can be converted into electric energy is attracting the spotlight as the alternative energy of the future in the respect that it is free of pollution and is of unlimited resource.
For these merits, solar electric power generation systems are utilized in automobiles, toys, residential generation of electricity, street lights, clock towers, unmanned lighthouses, and various communication facilities.
In general, a system of solar electric power generation includes solar cell modules, batteries, and electric transformers, though its components may change depending on the field of application, the load types, and plant site conditions. The solar cell modules convert sunlight into electric energy, the batteries store the electric energy generated from the solar cell modules, and the electric transformers convert the direct currents produced from the solar cell modules into alternating currents.
The biggest problem with conventional solid state solar cells is the relatively high cost due to the primary material, silicon, which is expensive and the processing of silicon is costly. To reduce the cost, a number of different approaches have been attempted over the last decade. One of the approaches is to make the layer of silicon, which is traditionally required to be relatively thick for reasonable photon capture rates, thinner. But to date, that approach has met with a variety of practical problems. Among other approaches attempted to overcome various limitations of the solid state semiconductor solar cells are the developments of organic film solar cells, also called polymer solar cells, and Dye-Sensitized Solar Cells (DSSCs), also called Grätzel's cell.
A DSSC is composed largely of a photo-anode, molecular dyes as sensitizers, a counter electrode, and an electrolyte. Upon a transparent electrode, a layer of porous nanomaterial is formed, which is covered with photosensitive molecular dyes. The layer of nanomaterial operates as the conventional n-type photo-anode for a silicon solar cell, as well as a scaffold material that provides huge surface area to hold thereupon large numbers of the dye molecules that generate photoelectrons in a three dimensional matrix, because the dye molecules are nanometer sized and a thick layer of dye molecules are needed to capture sufficient amount of incoming sunlight. Typically, titanium-dioxide (TiO2) nano-particles are used for such nanomaterial due to its known high efficiency and chemical stability. Above the layer of molecular dye, a layer of electrolyte, a liquid conductor, is formed and thereupon is placed a counter electrode, typically a metal electrode acting as the p-type cathode for a silicon solar cell.
When sunlight passes through the transparent electrode into the dye layer, the photons are absorbed by the photosensitive molecular dyes attached to the TiO2 nano-particles. Then an electron of the dye molecule becomes excited from a ground state to excited states (photoexcitation) to provide photoelectrons, which are then injected into the conduction band of the TiO2. In a traditional p-n junction semiconductor solar cell, the charge carriers (electrons and holes) move by drift, driven by electrostatic field established across the electrodes. By contrast, in a DSSC, the charge carriers (electrons) from the TiO2 move by diffusion to the transparent electrode according to the gradient of electrochemical potential or carrier concentration. The electrons then flow from the transparent electrode through an external circuit for powering external loads and re-enter the cell through the counter electrode, and into the electrolyte.
The injection of electrons from the dye molecules into the TiO2 does not introduce, like the traditional p-n type semiconductor solar cells, holes in TiO2, but only an extra electron. The dye molecule, which lost an electron and would decompose if another electron is not provided, strips an electron from an iodide, typically contained in the electrolyte, to oxidize it into a triiodide. The triiodide then mechanically diffuses to the counter electrode where it recovers an electron re-introduced into the cell through the counter electrode after flowing through the external circuit. The rapid repetition of such oxidization and reduction processes in a DSSC makes possible the generation of electric current from sunlight. Also, although it is possible in principle for the electrons from TiO2 to recombine into the dye molecules, which would inhibit photo-current, the rate at which this occurs is quite slow as compared to the rate the dye molecules regain electrons from surrounding electrolyte.
DSSCs are currently regarded as the most efficient third generation solar cells available and have the following advantages over conventional semiconductor solar cells. First, DSSCs are more economic than the conventional solar cells, such as silicon solar cells, for their manufacturing cost is only one-fifth of the latter while their energy conversion efficiency is only ten percent smaller than that of the latter. And unlike conventional solar cells, DSSCs can be made into light, thin, bendable, flexible, and transparent sheets, while mechanically robust, promising huge fields of commercial applications. Also, the conventional solar cells show a rapid reduction in operational efficiency as the temperature increases because they are encased in a glass box for protection from potential physical damage. By contrast, being surrounded by thin conductive layers that is structured to rapidly radiate heat away easily, DSSCs retain a relatively stable efficiency over a wide range of temperature. Further, for the conventional semiconductor solar cells, the energy conversion rate highly depends on the incidence angle of the sun light, which changes continuously as the sun moves. But DSSCs have less fluctuation in the energy conversion rate with the change of light incidence angle. Still further, DSSCs work even in low-light conditions such as under cloudy skies, while the traditionally designed silicon cells suffer a cut off of electricity at some lower limit illumination.
Accordingly, DSSCs have now become the subject of worldwide research and development efforts. Despite the numerous advantages, however, DSSCs have the drawbacks that they cannot be mass-produced due to the absence in the art of any manufacturing system, facilities, or teachings for a large-scale production. Typically, they have been produced only one by one at a time manually in the laboratories of either academia or industry.
To describe the conventional manufacturing method of DSSCs, in the case of the original Grätzel design, two glass substrates, front and back substrates, each typically sized 150 millimeters in width and length, are prepared and go through an isolation process in which a laser beam cuts grooves on a surface of each glass plate for locating electrodes and separating abutting cells. Next, along the edge of the back substrate, multiple orifices of about 3 millimeters in diameter are perforated successively by high-pressured streams of tiny sand powders, called sand master, so that when the two substrates are subsequently attached together, electrolyte liquid may be injected into the cell through the orifices. Then, the substrates go through ultrasonic facilities that eliminate impurities via ultrasound vibration. The next step is to form a TiO2 layer on the front glass substrate and a silver electrode along the groove cut on the substrate surface. Upon the back substrate, a platinum electrode is formed along the groove on its surface. The next step is to adhere molecular dye sensitizers by covalent bonding to the surface of the TiO2 nano-particles. For that, the front substrate with the TiO2 layer is immersed in a tank containing the dye material and a solvent. Typically, the dye material is sufficiently adhered to the TiO2 nano-particles when the substrate is left immersed in the tank for about four hours. Next, the dyed front glass substrate and the back substrate with the platinum electrode are attached together and sealed by applying heat, pressure, and sealants. Lastly, electrolyte liquid is injected through the orifices on the back substrate into the cell and the orifices are covered by glass.
Up to date, the manufacturing process of DSSCs in the art requires manual handling and implementation of each of the steps described above. Also, the manufacturing process demands manual transportation of cell components between successive steps. Even then, DSSCs can be produced only one by one at a time. Such process is tedious, inefficient, and quite costly and time consuming. Also, in the present manual manufacturing process in the art, it is very difficult to control the parameters in each manufacturing step to maintain or improve the quality of the product. Particularly, one of the biggest hurdles challenging the art in manufacturing DSSCs lies in the dyeing step. The present dyeing technology in the art necessitates immersion of the substrate with the TiO2 layer into a dyeing tank and waiting for about four hours until the dye molecules are sufficiently adhered to TiO2 nano-particles by covalent bonding. That not only prolongs the production time, but also greatly hinders introduction or devise of any smooth, seamless flow of manufacturing line of steps. Another serious problem recognized in manufacturing DSSCs is the sealing of the cells, arising from the use of liquid electrolyte that has a temperature stability problem. Especially when the temperature rises, the liquid expands and tends to leak out of the cell if the sealing is not tight enough. Further, since the electrolyte solution currently used for DSSC contains volatile organic solvents, a great care and need are required for its sealing.
Therefore, there is a strong need in the art for developing a method of mass-producing DSSCs on a large scale in a continuous flow of line of automated steps, which does not require manual handling in each step, so as to reduce cost and production time.
Also, there is a need in the art for developing a method of producing DSSCs in which the parameters of the cells and manufacturing steps can be easily and automatically controlled as desired to ensure even and improved qualities of the products.
Further, there is a need in the art for developing a method in the manufacturing procedures of DSSCs, by which the dyeing of substrate with the TiO2 layer can be done automatically, more rapidly, and in a continuous, seamless line of manufacturing steps without interrupting or prolonging other steps.
Still further, there is a need in the art for developing a method in the manufacturing procedures of DSSCs to ensure a tight sealing of the liquid electrolyte to prevent its undesired leakage.